1. Field of the Invention
The invention relates to optical waveform observing apparatus for observing the waveform of an optical pulse having a predetermined repetitive frequency.
2. Description of the Prior Art
One example of conventional means for observing the waveform of an optical pulse having a certain repetitive frequency uses a streak tube used to convert the variation with time of an optical pulse into a luminance distribution, such as a streak image on a phosphor screen.
FIG. 10 is a block diagram showing the arrangement of conventional optical waveform observing apparatus as disclosed in U.S. Pat. No. 4,645,918. In that apparatus, a hematoporphyrin derivative 104 is repetitively excited with a pulsed light beam repetitively outputted by a dye laser oscillator 101, to emit fluorescence, the waveform of which is observed with a streak tube 130. Streak tube 130, as shown in FIG. 10, comprises a photocathode 131 to which fluorescence is applied, an accelerating electrode 135 for accelerating the electron beam output by the photocathode 131, a focusing electrode 136 for focusing the electron beam accelerated by the accelerating electrode 135, an aperture electrode 137, deflecting electrodes 133 for deflecting in a sweep mode the forward electron beam after it passes through the aperture electrode 137, a microchannel plate 132 for multiplying the deflected electron beam and a phosphor screen 134 for receiving the output electron beam of the microchannel plate 132.
The pulsed light beam repetitively outputted by the dye laser oscillator 101 is divided into two pulsed light beams by a beam splitter 102. One of the two pulsed light beams excites the hematoporphyrin derivative 104 repetitively. The fluorescence emitted by derivative 104 is applied through an optical system 116 to the photocathode 131 of the streak tube 130 at a certain repetitive frequency. The other pulsed light beam is applied to a photodiode 105 to form an electrical signal TR for a predetermined deflecting voltage that is applied to deflecting electrodes 133 in streak tube 130. More specifically, the pulsed light beam is subjected to photo-electric conversion by the photodiode 105 to form the electrical signal TR. The electrical signal TR, being delayed by a time sweep circuit 107 under the control of a control unit 110, is converted into a deflecting trigger signal that is applied to a deflecting circuit 108. Circuit 108 in turn produces a deflecting voltage in synchronism with the deflecting trigger signal.
With respect to the streak tube 130, the output electron beam of the photocathode 131 is deflected by the deflecting voltage applied across the deflecting electrodes 133 in the sweep mode, so that the variation with time of the fluorescence applied to the photocathode 131 is converted into a spatial luminance distribution on the phosphor screen 134, i.e., it is observed as a streak image. In the optical waveform observing apparatus of FIG. 10, the light of the luminance distribution on phosphor screen 134, i.e., the light of the streak image, is applied through a lens 118 to a sampling board 111 with a slit 109, so that it is sampled by slit 109 to provide a sampling waveform. The sampling waveform is applied to a photomultiplier 112 that subjects it to photo-electric conversion and multiplication. The output of the photo-multiplier 112 is applied through an amplifier 113 to a display unit 114.
The operation of the optical waveform observing apparatus thus organized will be described with reference to FIG. 11 which is a time chart.
In response to the pulsed light beam repetitively outputted by the dye laser oscillator 101, the hematoporphyrin derivative outputs fluorescence with a repetitive period. This is indicated in part (a) of FIG. 11 which shows the waveform of an incident light beam "IN", which is applied to the photocathode 131 of the streak tube 130. Also in response to the pulsed light beam of dye laser oscillator 101, the photodiode 105 outputs the electrical trigger signal TR a shown in part (b) of FIG. 11. As is apparent from parts (a) and (b) of FIG. 11, the electrical trigger signal TR is completely synchronized with the incident light beam IN. The electrical trigger signal TR is gradually delayed by time sweep circuit 107 as shown in part (c) of FIG. 11, thus being converted into the deflecting trigger signal. As shown in part (c) of FIG. 11, at the n-th sampling time, the deflecting trigger signal TR1 is delayed by a period of time n.t from the trigger signal TR; at the (n+1)-th sampling time, the deflecting trigger signal TR2 is delayed by a period of time (n+1) t; at the (n+2)-th sampling time, the deflecting trigger signal TR3 is delayed by a period of time (n+2).t; and so forth, where t is the unitary delay time of the deflecting trigger signal.
When the deflecting trigger signals are applied to deflecting circuit 108, the latter produces the deflecting voltage V as shown in part (d) of FIG. 11. As seen in part (d) of FIG. 11, the deflecting voltage V is maintained at a potential V.sub.m when it is not used to deflect the output electron beam of the photocathode 131. However, when the deflecting trigger signal is applied to deflecting circuit 108, the deflecting voltage drops to a potential -V.sub.m substantially with a ramp characteristic, to deflect the electron beam. Whenever the deflecting voltage is decreased in the above-described manner, the electron beam is deflected downwardly, as a result of which it is observed as streak images FG1, FG2, FG3, . . . at the sampling times as shown in part (e) of FIG. 11. As a result, the variation in intensity of the incident light beam IN can be observed as a luminance distribution.
As seen from parts (a) and (e) of FIG. 11, the streak images FG1, FG2, FG3, reflect the respective incident light beams IN. However, the streak images are shifted in phase from one another because the deflecting voltages at the sampling times are shifted in phase from one another. When, during the electron beam deflecting operation, the deflecting voltage is near zero (0) volts, the electron beam reaches the central portion and emits a light beam at the central portion of the phosphor screen. Most of the light beam reached passes through the slit 109 of the sampling board 111, so that it is detected as a sampling signal by the photomultiplier 112. Referring to part (e) of FIG. 11, at the respective n-th, (n+1)-th and (n+2)-th sampling times, sampling signals P1, P2 and P3 are extracted by slit 109 of sampling board 111, so that the light beam is detected as a sampling signal by the photomultiplier 112.
The sampling signals Pl, P2, P3, . . . are detected in a time-series mode and arranged, as a result of which the pulse waveform of the incident light beam "IN" can be observed with a predetermined time resolution.
In the case of FIG. 10, the sampling board 111 with the slit 109 is provided outside of the streak tube 130. However, as shown in FIG. 12, the sampling board may be provided inside the streak tube. That is, the sampling board may be arranged between the microchannel plate 132 and the phosphor screen 134. In the streak tube 150 shown in FIG. 12, a sampling electrode 151 is provided therein and held at the same potential as the phosphor screen 134. The streak tube having the sampling electrode mounted inside is generally referred to as "a sampling streak tube". With respect to the sampling streak tube, the sampling waveform can be extracted before the electron beam reaches the phosphor screen 134. Therefore, unlike the apparatus shown in FIG. 10 in which the electron beam is applied to the phosphor screen 134 throughout the whole sweep period, in an optical waveform observing apparatus including the sampling streak tube, the necessary electron beam is applied only to the central portion of phosphor screen 134, and only a part of the luminance distribution is applied to the photomultiplier 112. As a result, the sampling waveforms detected are less affected by the background noises of the phosphor screen.
In the streak tube 130 shown in FIG. 10 or in sampling streak tube 150 shown in FIG. 12, the accelerating electrode 135 is a circular mesh accelerating electrode and therefore, the electron beam passing through the accelerating electrode 135 includes background noises. The mesh opening percentage of the accelerating electrode 135 is generally 60%. As shown in part (a) of FIG. 13, about 40% of the incident light beam which, not being subjected to photoelectric conversion by the photocathode 131, has passed therethrough, is scattered by the mesh material, thus forming scattered light beams DF. The scattered light beams DF are applied to the photocathode 131 from behind, thus causing the photocathode 131 to emit photoelectrons BG that represent the background noises. Part (b) of FIG. 13 shows the density distribution of the electrons that reach the sampling electrode 151 in the streak tube 150. As is apparent from part (b) of FIG. 13, in the case where the accelerating electrode 135 is a mesh electrode, the electron density distribution .rho. includes a signal electron density distribution .rho..sub.o and a scattered light beam photoelectron density distribution .rho..sub.e. The latter density distribution .rho..sub.e represents the background noises and lowers the sampling waveform observation accuracy. As seen from the density distribution .rho..sub.e, the photoelectrons BG, due to the scattered light beams, are greatest in number at the central portion of the phosphor screen to which the incident light beam is applied, and lower in number towards the periphery of the phosphor screen. The scattering electrons around the incident light beam lowers the measurement accuracy. As a result, it is difficult to measure a light pulse waveform with high time resolution, since background noises due to the scattered light beams DF are produced before and after the measured light pulse.
The background noises as represented by the density distribution .rho..sub.e, is spread over a wide range. As a result, during a non-sweep standby period, while the incident light beam is applied to the photocathode 131 the noise electrons, of density distribution .rho..sub.e will be sampled with high probability. Accordingly, that noise electrons should be eliminated. For this purpose, in the conventional optical waveform observing apparatus, the deflecting voltage V applied by the deflecting circuit 108 is made large in amplitude and high in through rate (voltage/time). This can lower the probability that, during the sweep of the electron beam, the photoelectrons attributable to the scattered light beams are sampled.
As the amplitude of deflecting voltage V is made large, the potential V.sub.m of the deflecting voltage V is large during the non-sweep stand by period. Therefore, even if the incident light beam is applied to the photocathode 131, the signal electrons attributable to the incident light beam are greatly deflected by the deflecting electrode 133, thus reaching a peripheral portion of the phosphor screen 134 that is remote from the central portion. As a result, even if the density distribution .rho..sub.e of the photo electrons due to the scattered light beams is spread over a wide range, the probability that these photoelectrons reach the central portion of phosphor screen 134 is small and, accordingly, their effect on the sampling waveform is eliminated.
With respect to the sampling streak tube 150 shown in FIG. 12, the potential V.sub.m of the deflecting voltage V is made so large that, during the non-sweep period When the electron beam is not undergoing deflection, the electron beam reaches a portion of the sampling electrode 151 that is remote from the slit. Accordingly, in this case also, the sampling waveform is protected from being affected by the photoelectrons caused by the scattered light beams. In practice, the potentials V.sub.m and -V.sub.m of the deflecting voltage V are about +1 KV and -1 KV, respectively.
In the optical waveform observing apparatus in which the waveform of an incident light beam is observed by sampling, an aperture time .DELTA.t, during which the sampling waveform is extracted, is determined by a sweeping velocity v.sub.t at which the electron beam emitted from the photocathode 131 in response to the incident light beam is deflected to sweep across the slit of the sampling electrode, a diameter u of the electron beam, and a slit width w of sampling electrode 151. The aperture time .DELTA.t corresponds to the time resolution and is represented by the following equation (1): ##EQU1## The sweeping velocity v.sub.t can be represented by the following equation (2): EQU v.sub.t =S.times.T (2)
where S is the deflection sensitivity (cm/V) of the sampling streak tube 150, and T is the through rate (V/sec) of the deflecting voltage.
As is apparent from equation (1) and (2 ), the aperture time .DELTA.t is decreased as the deflection sensitivity S and/or the through rate T increase, with the result that the time resolution is improved.
However, the conventional optical waveform observing apparatus experiences difficulties when the distance between the pair of deflecting electrodes 133 is decreased for improvement of the deflection sensitivity S: Since the deflecting voltage V.sub.m has been made large in amplitude for the reasons described above, the greatly deflected electron beam will, during the non-sweep period, strike against and be reflected by the deflecting electrodes and the reflected beam will travel to the central portion of the phosphor screen 134. Accordingly, in the conventional optical waveform observing apparatus, it is impossible to greatly reduce the distance between the pair of deflecting electrodes 133. That is, the possible improvement to the deflection sensitivity S is limited. As a result, it is impossible to obtain a time resolution on the order of several pico-seconds and, further, it has been impossible to increase the repetitive frequency of the deflection.